The United States is packed with astronomical observatories — located on high mountain tops or splayed across a desolate desert. Beyond their beautiful terrestrial settings, many of these observatories have fueled the most important astronomical discoveries of the last century.

The world’s largest on-land optical telescope is at the W. M. Keck Observatory atop Hawaii’s nearly 14,000-foot Mauna Kea peak. However, this US observatory is neither open to visitors nor easy to work into a continental road trip. Rather, this road trip begins in California — where the Big Bang Theory was developed — and takes you across several western states, where a dry climate and tall elevations provide the best observing conditions.

The road trip also includes a handful of eastern observatories. These can be part of your cross-country astro road trip, or you can split your travel into a western and eastern tour. However you choose to explore, these are the best US observatories you can visit on a trip across the country.